This invention relates to ultrasonic systems and, more particularly, to apparatus for imaging sections of a body by transmitting ultrasonic energy into the body and determining the characteristics of the ultrasonic energy reflected therefrom.
During the past two decades ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of subcutaneous blood vessels including imaging of smaller blood vessels.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as a lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signal representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in synchronism with the time-base signals produced a continuous display, called an "A-scan", in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scan lines on the display, and successive transverse positions are utilized to obtain successive scan lines on the display. This type of technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape.
While successes have been achieved in the field of ultrasonic imaging, there are a number of problems which need to be overcome in obtaining high quality ultrasonic images in a convenient, reliable and cost-effective manner. Regarding problems which have been partially overcome, it is known, for example, that ultrasound is almost totally reflected at interfaces with gas. This has led to the use of coupling through a fluid such as water or the use of a direct-contact type of transducer. The latter technique may give rise to problems when attempting to image structures such as arteries which may be only a few millimeters below the skin surface, the contact imaging causing aberrations in the near field of the transducer. Coupling through a fluid offers advantage over direct-contact in this respect, but leads to various design problems and cumbersome generally non-portable structures which are inconvenient to use, especially when attempting to register them accurately on a patient. Some techniques involve immersing the patient in water or obtaining appropriate contact of the body part with a bulky water tank wall.
The need to scan the ultrasonic beam in two dimensions has added to the problems of conciseness and cost-effectiveness. One class of techniques utilizes electronic beam steering wherein a multi-element array is utilized in conjunction with dynamic focusing circuitry employing variable delay elements. This technique is generally complex, especially when used to achieve scanning in two dimensions. Also, at the necessary bandwidths, it is found that the delay elements in such circuitry, including numerous fixed delay elements, become physically cumbersome and add cost, size, and weight to the scanning units.
The above factors, inter alia, contribute to the difficulty in obtaining an ultrasonic imaging apparatus which is convenient to use, portable, relatively cost-effective, and has these attributes without sacrificing necessary operational characteristics. It is among the objects of this invention to provide such an apparatus and to offer solution to prior art problems as set forth.